According to the state of the art, automatic transmissions, in particular for motor vehicles, comprise planetary gear sets that are shifted by means of friction elements or shifting elements and are typically connected to a start-up element that is subject to a slip effect and is alternatively provided with a lock-up clutch, such as a hydrodynamic torque converter or a fluid clutch.
Such an automatic transmission is known, for example, from DE 199 12 480 B4 of the applicant. It comprises three single-bar planetary gear sets along with three brakes and two clutches for the shifting of six forward gears and one reverse gear, one drive shaft and one output shaft, whereas the bar of the first planetary gear set is constantly connected to the ring gear of the second planetary gear set and the bar of the second planetary gear set is constantly connected to the ring gear of the third planetary gear set and the drive shaft is connected to the sun gear of the second planetary gear set.
In addition, it is provided with the known transmission that the drive shaft is connectable through the first clutch to the sun gear of the first planetary gear set and through the second clutch to the bar of the first planetary gear set, whereas the sun gear of the first planetary gear set is connectable through the first brake to a housing of the transmission and the bar of the first planetary gear set is connectable through the second brake to the housing of the transmission, whereas the sun gear of the third planetary gear set is connectable through the third brake to the housing of the transmission. The output shaft of the transmission is constantly connected to the bar of the third planetary gear set and the ring gear of the first planetary gear set.
A 9-gear multi-speed transmission is also known from DE 29 36 969 A1; it comprises eight shifting elements and four planetary gear sets, whereas one planetary gear set is used as an upstream transmission and the main transmission features a Simpson set and an additional planetary gear set used as a reversing gear.
Additional multi-speed transmissions are known, for example, from DE 10 2005 010 210 A1 and DE 10 2006 006 637 A1 of the applicant.
In general, automatically shiftable vehicle transmissions in planetary design are already described in the state of the art, and are subject to continuous development and improvement. Thus, such transmissions should require low construction costs, in particular a small number of shifting elements, and, upon a sequential shifting operation, should avoid double gearshifts, i.e. the switching on and/or off of two or more shifting elements such that, during gearshifts in defined gear groups, only one shifting element is changed.
A multi-speed transmission in planetary design is known from DE 10 2008 000 428 A1 of the applicant; this features an input and an output, which are arranged in a housing. With the known transmission, at least four planetary gear sets, hereinafter referred to as the first, second, third and fourth planetary gear sets, at least eight rotatable shafts—hereinafter referred to as the input shaft, output shaft, third, fourth, fifth, sixth, seventh and eighth shafts—along with at least six shifting elements, comprising brakes and clutches, are provided; their selective meshing brings about different transmission ratio relationships between the input and the output, such that, preferably, nine forward gears and one reverse gear can be realized.
Thereby, the first and the second planetary gear sets, which are preferably formed as negative planetary gear sets, thus with a negative stationary transmission ratio, a shiftable upstream gear set, whereas the third and the fourth planetary gear sets form a main gear set.
With the known multi-speed transmissions, it is provided that the bars of the first and the second planetary gear set are coupled to each other through the fourth shaft, which is connected to one element of the main gear set that the ring gear of the first planetary gear set is coupled to the sun gear of the second planetary gear set through the eighth shaft, which is detachably connectable to the drive shaft through a first clutch and that the sun gear of the first planetary gear set is attachable to the housing of the transmission by means of the third shaft through a first brake, and is detachably connected to the drive shaft through a second clutch, whereas the ring gear of the second planetary gear set is attachable to a housing of the transmission by means of the fifth shaft through a second brake. In addition, the seventh shaft is constantly connected to at least one element of the main gear set and is attachable to the housing of the transmission through a third brake, whereas the sixth shaft is constantly connected to at least one additional element of the main gear set and is detachably connectable to the drive shaft through a third clutch; the output shaft is constantly connected to at least one additional element of the main gear set.
Preferably, with the known transmission, the fourth shaft is constantly connected to the ring gear of the third planetary gear set, whereas the sixth shaft is constantly connected to the ring gear of the fourth planetary gear set and the bar of the third planetary gear set, and is detachably connectable to the drive shaft through the third clutch. In addition, the seventh shaft is constantly connected to the sun gears of the third and fourth planetary gear set, and is attachable to a housing of the transmission through the third brake. Thereby, the output takes place through the output shaft constantly connected to the bar of the fourth planetary gear set. Furthermore, the third and the fourth planetary gear set can be assembled or reduced to a Ravigneaux set with a common bar and a common ring gear.
According to the state of the art, the shifting elements of the multi-stage transmissions designed in such a manner, which are typically designed as multi-disk clutches or brakes, are hydraulically actuated, which feature high hydraulic losses in a disadvantageous manner. In order to avoid these actuating losses, the use of actuated shifting elements that are in line with demand would be particularly advantageous.
With “shifting elements in line with demand,” shifting elements that, no energy or less energy to maintain their shifting state than upon the change to the shifting state are particularly understood.
In order to enable the use of actuatable shifting elements in line with demand, the shifting elements, in particular the clutches, should be easily accessible from the outside.